An ion pump (also referred to as a sputter ion pump) is a type of known vacuum capture pump capable of reaching pressures as low as 10−11 mbar under ideal conditions. Ion pumps do not mechanically pump gases out of a chamber, but rather function by converting gases within a chamber to solids that are then deposited on surfaces within the ion pump, as well as through physical sorption of gases (particularly noble gases) on surfaces within the ion pump. According to the law of ideal gasses, the pressure inside of a fixed volume at a fixed temperature is proportionate to the number of gas molecules present. Therefore, by capturing gas molecules and converting or binding them to solids, the gas pressure inside the chamber is reduced. An ion pump is a device that ionizes gas within a vessel (to which the ion pump is attached) and employs a strong electrical potential, typically 3 kV to 7 kV, that allows the gas ions to accelerate into and be captured by a solid electrode and its residue.
An ion pump normally includes an anode formed from an array of close packed, axially symmetric (tubular) metal elements and a cathode surface normal to the axis of the tubular array and spaced apart from the anode. The cathode(s) and anode(s) are disposed within a hermetic sealed housing. The cathode is a chemically active, non-magnetic metal, typically titanium, vanadium, tantalum, or zirconium. The anode is frequently stainless steel. The cathode typically faces open ends of the tubular anodes.
An ion pump normally typically utilizes a static magnetic field in combination with the electric field inside the ion pump to enhance ionization of gasses inside the pump. In FIG. 1, a perspective view of an example of an implementation of a known ion pump 100 is shown.
The electric potential can be generated using a set of three electrodes: a ring 102 and two end-caps 104 and 106 between a magnet 108. In this example, the ring 102 is an anode element, such as a cylindrical anode of stainless steel, and the end-caps 104 and 106 are cathodes. For trapping of ions, the end-cap electrodes 104 and 106 can be kept at a negative potential relative to the cylindrical anode 102. This potential produces a saddle point which traps ions along the trap axial direction 110. The electric field causes ions to oscillate along the trap axis 110. The magnetic field in combination with the electric field causes charged particles to move in the radial plane 112 with a motion which traces out a helix.
In FIG. 2, a side-view of the ion pump 100 of FIG. 1 is shown in combination with the vessel 200 that has an inlet 202. The cathode plates 104 and 106 are shown positioned on both sides of one of the anode cylinders 102. It is appreciated that while only one anode cylinder 102 is shown for convenience, the description extends to a plurality of anode cylinders 102. Typically, the anode 102 is made of stainless steel, aluminum or other similar metals, which serves as a “gettering” material. A magnetic field 204 is oriented along the axis 206 of the anode 102. Electrons 208 are emitted from the cathodes 104 and 106 due to the action of an electric field 210 and, due to the presence of the magnetic field 204, the electrons 208 move in long helical trajectories 212 which improves the chances of collision with gas molecules 214 inside the ion pump 100 that are introduced via the inlet 202.
The usual result of a collision of a gas molecule 214 with an electron 208 is the creation of a positive ion 216 that is accelerated to some voltage potential by the anode voltage and moves almost directly in the direction 218 to the cathode 106. The influence of the magnetic field 204 on the ion is relatively small because of the ion's relatively large atomic mass compared to the electron mass.
Cathodes 104 and 106 may be of titanium (tantalum, other related alloys, or other getter metals). In the case of cathodes 104 and 106 being made of titanium, ions 216 impacting on the titanium cathode surface sputter titanium atoms (or molecules) 220 in a direction 222 away from the cathode 106, thereby forming a getter film on the neighboring surfaces and stable chemical compounds with the reactive or “getterable” gas particles (e.g. CO, CO2, H2, N2, O2). This pumping effect is very selective for the different types of gas molecules 214 and is the dominating effect with ion pumps. The number of sputtered titanium molecules 220 is proportional to the pressure inside the ion pump. The sputtering rate depends on the ratio of the mass of the bombarding ions 216 and the mass of the cathode material 220.
In an example of operation, electrons 208 are temporarily stored in the anode region 224 of the ion trap 100. These electrons 208 ionize incoming gas atoms and molecules 214. The ions 216 are accelerated to strike the chemically active cathodes 104 and 106. On impact, the accelerated ions 216 will either become buried within the cathode 104 and 106 or sputter cathode material 220 onto the walls of the ion pump. The freshly sputtered chemically active cathode material 220 acts as a getter that then evacuates the gas by both chemisorption and physisorption resulting in a net pumping action.
Both the pumping rate and capacity of such capture methods are dependent on the specific gas molecules 214 being collected and the cathode material absorbing it. Some gas molecules 214, such as carbon monoxide, will chemically bind to the surface of a cathode material. Others, such as hydrogen, will diffuse into the metallic structure.
U.S. Pat. No. 7,850,432 (the entire contents of which are incorporated herein by reference) describes various ion pump designs having a gastight housing with a gas inlet that is attached to a vacuum chamber and having an anode, a cathode, and a complex blocking shield assembly.